The present invention is directed to high-frequency modulation of semiconductor lasers, and, more particularly, to the use of electric fields applied in a direction normal to the direction of current flow in the quantum wells of a semiconductor laser to modulate such a laser through energy level shifting.
Semiconductor lasers have several important features, including extreme monochromaticity and high directionality (or time and space coherence, respectively), which features are generally similar to those of other lasers. However, solid state junction lasers differ from other lasers in several basic respects. First, the quantum transition of a solid state junction laser occurs between energy bands (conduction and valence bands), rather than between discrete energy levels, as is the case in the solid state ruby laser. Further, the size of a solid state junction laser is very small, and its characteristics are strongly influenced by the properties of the junction material, such as doping, and band tailing. Population inversion in a solid state junction laser takes place in the very narrow junction region, and the pumping is supplied by a forward bias across the junction. Recently, semiconductor devices known as quantum well lasers have attracted technological interest due to their low threshold currents, greater insensitivity of threshold current with respect to temperature, and potential fiber optic communication applications. A quantum well may be defined as a restricted area within the transition layer of the junction where electrons form standing waves, with the width of the quantum well determining the energy level of the electrons, and thus controlling the frequency of light emitted by the laser device.
Quantum well lasers exhibit novel properties, compared to those of conventional lasers, and these properties have useful applications. For example, the high density of states at the quantum well band edge can be exploited to give very low threshold currents. However, the problem of optimizing the design of a device becomes complex, because many more parameters are involved than in conventional semiconductor lasers. For example, the well width, the number of wells, and the composition and thickness of the layers separating the wells must all be taken into consideration. Known quantum well lasers consist of a very thin layer (on the order of less than 500 Angstroms) of a low band-gap semiconductor material sandwiched between higher band-gap semiconductor layers. The resulting energy band profile for such a device results in the confinement of the electric carriers and the formation of discreet energy states. It was shown by E. E. Mendez, et al. ("Effect of Electric Field on the Luminescence of GaAs Quantum Wells," Physical Review, Vol. 26, no. 12, Dec. 15, 1982, pp. 7101-7104), that significant effects on the carrier confinement and energy states of the quantum well could be produced by means of external parameters such as an electric field applied perpendicularly. It was shown that the field polarized the electron and hole distribution to opposite sides of the quantum well along the direction of the field, or against the direction of the field, depending on the sign of the charge, with the result that the quantum states experienced an energy shift. It was proposed by Mendez, et al., that such effects could be used for control and modulation of the photon energy and the intensity output of a radiation-emitting device based on quantum wells.
It was also shown by Yamanishi, et al. (Japanese Journal of Applied Physics, Vol. 22, No. 1, January 1983, pp.L22-L24), that a direct modulation of the perpendicular electric field in such a quantum well laser could be used to modulate the light output intensity. Yamanishi, et al., proposed a field effect laser, or light-emitting diode, where the photon emission rate was controlled by a gate voltage. This control was obtained mainly through changes in spatial distributions of the carriers, or, what is the same, by the quantum mechanical size effect modulations for the wave functions of the carriers, while keeping the carrier concentration per unit area substantially constant.
Although such devices have represented significant steps in the development of this art, problems have been discovered in operating such devices at microwave frequencies. First of all, devices such as those proposed by Yamanishi, et al., require high electric fields to achieve a significant reduction in gain, particularly in the case of narrow quantum wells. This is because an electric field, in reducing the subband energies causes more charge to flow into the quantum well, and this serves to increase the carrier density and the peak gain. In addition, undesirable leakage current conduction through the semi-insulating barrier layer or the tunnelling of carriers to the control gate used to provide the electric field, both can pose serious problems at the high fields required to modulate a narrow quantum well laser. Thus, such prior devices require large microwave frequency currents to produce modulation and a large displacement current within the device itself. At frequencies of 40 GHz or more, current densities on the order of several thousand amperes per square centimeter would be needed to modulate a single quantum well laser.